Since polyhexamethylene adipamide fibers are superior in tenacity, toughness, heat-resistance, dyeability and coloration, they are broadly used as fibers for industrial materials, interior cloth, bed cloth and clothing. Especially, on account of their excellent tenacity, toughness, heat resistance, fatigue resistance and adhesion to rubber, the polyhexamethylene adipamide fibers are broadly used as fibers for tire cords.
Recently an energy saving technology is required of tires and tires which can save driving fuel are desired. For this reason, tire makers are pursuing tires which have lower rolling resistance and are lighter. Thus tire cords having higher modulus and higher tenacity are required, too. Particularly, polyamide tire cords are mainly used for tires of large size with a number of plies of embedded fabrics, i.e., for light trucks, truck-buses, construction vehicles, airplanes and the like. Accordingly, there is a problem that the number of yarns employed per tire is large. Reduction in the number of plies or ends of embedded fabrics can achieve not only saving of fuel due to lightening of tires but also improved fatigue-resistance due to decrease in heat-generation and increase in heat-exhaustion, improved safety for separation due to improved adhesiveness and improved productivity in the manufacturing process of tires. Thus, higher tenacity polyamide tire cords are demanded. At present, polyamide fibers on sale have a tenacity of 9.0 to 9.5 g/d. Although many attempts to make stronger polyamide fibers are continued, satisfactory results have not been obtained yet.
Generally, in order to obtain high tenacity polyamide fibers or polyester fibers, polyamide polymer or polyester polymer having a high degree of polymerization must be spun into fibers and subsequently the spun fibers must be drawn at a high draw ratio. However, the melt viscosity of extruding polymers increases with increased degrees of polymerization of polymers, and as a result, the degree of orientation of spun fibers thus obtained increases and the stretchability of the spun fibers decreases. This feature is remarkable especially with polyhexamethylene adipamide whose crystallizing speed is notedly high.
On the other hand, Japanese patent application Kokoku No. 26207/1965 discloses a direct melt-spinning method for producing high tenacity Nylon fibers which comprises drawing polyhexamethylene adipamide spun fibers having a low degree of orientation in multiple steps. Furthermore, in order to obtain spun fibers having a low degree of orientation Japanese patent application Kokoku No. 7251/1964 proposes a method for controlling the atmospheric temperature below the spinning nozzle mounted on a spinhead in melt-spinning by providing a heating cylinder on the surface of the nozzle. By using these methods, the degree of orientation of spun fibers can be decreased and the spun fibers can be drawn at a high draw ratio and as a result, the tenacity of the drawn fibers is increased. Thus, with polyhexamethylene adipamide the tenacity of tire cords has been improved from 8 g/d to 9.0-9.5 g/d.
As the result of studies by the present inventors to increase the degree of polymerization, to attain high draw ratio and to obtain high tenacity of drawn fibers it has been found that drawn fibers having a tenacity of 10 g/d or more can be obtained. However, it has been found that the tenacity of such high tenacity drawn fibers having been subjected to twisting, weaving, adhesion-heat-treatment and vulcanization and subsequently having been taken out from the rubber is about 7 g/d which is the same as the tenacity of the commercially available polyhexamethylene adipamide fibers having a tenacity of 9.5 g/d having been subjected to the above described steps and subsequently having been taken out from the rubber. Also it has been found that decrease in tenacity of the fibers is remarkable in the vulcanization step, and that the attained effect on increasing the tenacity of the drawn fibers is not maintained at all. As the result of studies on high tenacity polyhexamethylene adipamide fibers having a less reduced tenacity in the after-treatments, especially in the vulcanization step, it has been found that increase in the thermal stability of elastic modulus of drawn fibers is very important.